medical implantable lead

ABSTRACT

The present invention relates to a medical implantable lead having a coaxial structure, where an insulating tube arranged between an inner coil and an outer coil is provided with a periodically alternating capacitance along the length thereof in order to reduce lead tip heating during MRI scanning.

FIELD OF THE INVENTION

The present invention relates to a medical implantable lead having anelongate coaxial arrangement of an outer coil, an inner coil and anintermediate insulating tube arranged between the outer coil and theinner coil.

BACKGROUND OF THE INVENTION

Magnetic Resonance Imaging (MRI) is of great use for generating an imageof the internal tissues of a human body. However, for persons who have amedical implantable lead implanted in their body, there are problemswith induced currents in the medical implantable lead causing, in turn,heating of the lead, in particular the distal tip of the lead. The MRIis based on Nuclear Magnetic Resonance (NMR) for protons of hydrogennuclei. It is well-known that all nuclei have spin which are randomlyoriented. When a magnetic field is applied, the proton spins becomeeither parallel or anti-parallel, and the energy levels are split into ahigher level for the anti parallel spin and a lower level for theparallel spin. Furthermore, the protons start precessing around themagnetic field direction with a precession frequency (Larmor frequency)which is proportional to the magnetic field, and with a precessionangle, which is also called flip angle. If an external pulsed RF signalwith Larmor frequency is applied, protons from the lower energy levelwith parallel spin will be excited to the higher energy level. Thisimplies that the precession angle will change and all protons willprecess in phase. After some time (in the order ms) the protons start torelax, that is the protons in the higher anti-parallel spin level willfall back to the lower parallel level, which implies that the precessionangle falls back to the original value, and at the same time the protonswill also de-phase. Both these processes will proceed with slightlydifferent time-constants. The MRI takes advantage of the relaxation andtime-constants to identify the substances in a human body. In-vitro MRIexperiments have shown that the implanted lead acts like an antenna andreceives the pulsed RF signal of the MRI scanning equipment. Thereception of the RF energy results in an RF wave propagating along thelead and heating the pacemaker lead tip to an unacceptable level. Someother parts of the lead become heated as well, although not as much asthe tip.

Referring to an in-vitro set up, where a particular gel is used tosimulate human tissue, the mechanisms for the RF energy transfer areidentified as follows. As mentioned above the precession frequency isproportional to the magnetic field, and more particularly at 42.58MHz/T. Currently most MRI devices operate at 1.5 Tesla, while 3 TeslaMRI devices are expected to increase. Thus, the frequency of the RFpulses, or RF wave, produced in a 1.5T MRI device is about 64 MHz. TheRF wave first passes through the boundary between the air and the gel.The RF wave undergoes a speed reduction from the speed in air v₀ to aspeed in the gel (human body) v₁ due to the dielectric constant (∈) ofthe gel, where v₁=v₀/sqrt(∈). The wavelength λ is also reduced by thesame factor, i.e. λ₁=A₀/sqrt(∈). The dielectric constant of the humantissue on average is in such an order that the resulting wave length inhuman tissue becomes close to the physical length of a typical medicalimplantable lead, e.g. in the order of half a meter. This transforms apacemaker lead to a good antenna. The RF energy is picked up by theouter coil of the lead, and then transferred to the inner coil via theinter-coil capacitance. This coaxial structure of the lead in fact is atransmission line, and the potential difference along the lead andbetween the outer and inner coils cause the above-mentioned propagation.The RF energy is eventually transferred to the lead tip, causing heatingof the tip.

This problem of lead tip heating has been addressed in prior art, suchas in US 2008/0033497 A1, where different solutions have been suggested.According to one solution, the inner and outer coils are wound inopposite directions and they are interconnected at their ends. Thepurpose is to reduce the total current. However, a probabledisadvantageous effect is that the current direction is determined bythe incident wave phase and a different winding direction will notchange the current direction. In fact, when studying the currents at acertain point they are always oppositely directed in the outer and innercoils irrespective of the winding direction. According to anothersolution, RF blocking circuits are inserted at half wavelength. Thiswould work, but it is difficult to realize such a lead structure.According to yet another solution the lead coils are arranged such as tocreate resonance circuits. Such a resonance property, which is of adistributed kind, is sensitive to lead configuration, which can lowerthe impedance at RF frequency. This may work to a certain extent, butthe outcome in each individual case is uncertain.

WO 2007/047966 also aims at providing a solution to the problem of tipheating, however in a lead structure where the conductors are notprovided in a coaxial structure with inner and outer coils but arrangedin parallel. Either the conductors are straight and parallel,individually and partially wound and parallel, or co-wound while stillparallel. Capacitors are arranged to interconnect the conductors. Thecapacitors are arranged at regular or irregular distances from eachother. By means of the capacitances a high impedance circuit isobtained, which appropriately tuned reduces the coupling of the pulsedRF signal to the lead. Furthermore, the high capacitance valuesmentioned in the WO document are most difficult to realize in a thinlead.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lead structurethat alleviates the above-mentioned drawbacks of the prior art andreduces the induced current of the lead due to the RF wave.

This object is achieved by a medical implantable lead according to thepresent invention as defined in claim 1.

The invention is based on an insight that a periodic capacitivestructure is capable of acting as a current reducer in a medicalimplantable lead subjected to a current inducing RF wave.

Thus, in accordance with an aspect of the present invention, there isprovided a medical implantable lead, comprising an elongate coaxialarrangement of an outer coil, an inner coil and an intermediateinsulating tube arranged between the outer coil and the inner coil,wherein the intermediate insulating tube has a periodically alternatingcapacitance along the length thereof.

Due to the alternating capacitance provided by the insulating tube thecurrent induced by the RF wave in the lead will be substantiallyreduced, and thereby the heating of the lead tip will be negligible.

In accordance with an embodiment of the medical implantable lead, theintermediate insulating tube comprises a set of at least two segments,which set of at least two segments is repeated along the length of theintermediate insulating tube, and which segments have differentcapacitances, thereby providing the periodically alternatingcapacitance. Combining segments of different capacitances to form theintermediate tube provides for flexibility in choosing the capacitancevalues of the respective segments of each set. In an embodiment thereare three segments in each set, i.e. three different capacitance valuesare repeated along the length of the intermediate tube.

In accordance with an embodiment of the medical implantable lead, thelength of each segment is shorter than or equal to 1/10 of the totallength of the intermediate insulating tube. This is true at least fortubes, and thus leads, having a length in the order of the wavelength ofan RF wave that it is exposed to, which in turn is the case when theinitially described problem of induced currents is at hand.

In accordance with an embodiment of the medical implantable lead, thesegments are individual parts which are arranged in engagement with eachother. This includes many different kinds of engagement, such as forinstance the segments abutting on each other, and the segments beingadhesively attached to each other.

In accordance with an embodiment of the medical implantable lead, adopant is used to tailor the capacitance values of the segments. Herethe intermediate insulating tube is made of silicone, where at least onesegment in each set is doped with a dopant. An appropriate choice of thedopant is BaTiO₄.

In accordance with an embodiment of the medical implantable lead, thecapacitance is varying continuously and periodically along the length ofthe intermediate insulating tube. In contrast to the embodiment whereindividual segments are mounted in a series, here the tube can be keptin one piece, while varying the capacitance in other ways along thelength of the tube.

These and other aspects, features, and advantages of the invention willbe apparent from and elucidated with reference to the embodimentsdescribed hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail and with reference tothe appended drawings in which:

FIG. 1 is a schematic side view of a medical implantable device;

FIG. 2 is a schematic longitudinal sectional view of a portion of anembodiment of a medical implantable lead according to the presentinvention;

FIG. 3 is an equivalent circuit diagram of the LC-structure of the leadin FIG. 2;

FIG. 4 is a current attenuation diagram resulting from a simulation ofan LC-structure with a periodically alternating capacitance;

FIG. 5 is a table illustrating the effect of parameter variations;

FIG. 6 is a schematic longitudinal sectional view of a portion ofanother embodiment of a medical implantable lead according to thepresent invention; and

FIG. 7 is a schematic longitudinal sectional view of a portion of yetanother embodiment of a medical implantable lead according to thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a typical implantable medical device 1 comprises animplantable lead and an electric unit, such as for instance a pacemaker,5. The implantable lead has a lead tip 7, which is to be connected tobody tissue.

A first embodiment of the medical implantable lead 3 according to thepresent invention comprises an elongate coaxial arrangement 3 a, aportion of which is schematically shown in FIG. 2. The coaxialarrangement 3 a extends between a proximal end 3 b of the medicalimplantable lead 3, where it is connected with the electrical unit 5,using the coaxial arrangement to send signals to and/or receive signalsfrom a desired part of the body, and a distal end 3 c of the lead, whereit is connected with the lead tip 7, which interacts with the bodytissue. Since this overall structure of a typical medical implantablelead is well known to the person skilled in the art, no furtherexplanation thereof will be set forth here. The coaxial arrangement 3 acomprises an outer coil 9, an inner coil 13, and an intermediateinsulating tube 11, arranged between the outer coil 9 and the inner coil13. The intermediate insulating tube 11 has a periodically alternatingcapacitance along the length thereof. Thereby it is possible to obtain aband-stop filter, wherein the capacitive intermediate insulating tube 11interacts with the outer and inner coils 9, 13, which are inductive, toform an LC structure having band-stop filter characteristics, as will befurther explained below. In this embodiment, the alternating capacitanceis provided by individual segments 15, 17 of tubular insulatingmaterial, which segments have one or the other of two differentcapacitances, a lower capacitance c₀ and a higher capacitance c₁. Thestructure of the intermediate insulating tube 11 can be regarded asconsisting of a series of two segment sets, wherein each set of twosegments 15, 17 consist of a first segment 15 having the lowercapacitance c₀ and a second segment 17, having the higher capacitancec₁. Thus, the set of two segments 15, 17 is repeated along the length ofthe intermediate insulating tube 11, which means that the segments 15,17 alternately have the lower c₀ and the higher c₁ capacitance along thelength of the tube 11. The segments 15, 17 are all of equal length,which is preferred although not necessary. As mentioned above, thesegments 15, 17 are individual parts, and they are arranged inengagement with each other. More particularly, the segments 15, 17 abuton each other. However, other kinds of engagement are also possible,such as adhesive attachment, and the like. The length of each segment15, 17 preferably is no longer than about 1/10 of the total length ofthe intermediate insulating tube 11 for typical medical applications.Some criteria for dimensioning the coaxial arrangement 3 a will now bedescribed.

First it is important to point out that only the RF field componentsperpendicular to the axial direction of the medical implantable lead 3contribute to the RF wave propagation in the lead 3.

The coaxial nature of the medical implantable lead 3 implies that thereare certain limitations for the energy propagation in the lead 3. Acoaxial structure is characterized by the per unit length inductance ofthe outer and inner coil and the per unit length inter-coil capacitance.Microwave theory tells us that the maximum speed v_(t) of a transmissionline is 1/sqrt(l_(u)*c_(u)), where l_(u) is the series inductance perunit length and c_(u) is the parallel capacitance per unit length. Inorder to exemplify the LC-structure of the lead 3, a typical pacemakerlead is considered. A typical total lead length for such a lead isapproximately 0.5 m. Now assume that the unit inductance per meter is 20μH and the unit capacitance per meter is 60 pF. Then,

v _(t)=1/(sqrt(l _(u) *c _(u))=1/(20*10⁻⁸*60*10⁻¹²)=0.28*10⁸ m/s,

which is the maximum speed for a wave which can be transported throughthis transmission line.

In an in-vitro test set-up, as referred to above, using a gel tosimulate body tissue, the speed of the wave (in the tissue) can becalculated easily.

V _(g) =c/sqrt(∈_(r)),

where v_(g) is the speed of the wave in gel,c is the speed of light in vacuum and∈_(r) is the relative dielectric constant.

The relative dielectric constant of the gel is ∈_(r)=81, which resultsin

V _(g)=3*10⁸ /sqrt(81)=0.33*10⁸ m/s,

which is roughly equal to the calculated maximum wave speed v_(t) in thetransmission line. Consequently, theoretically, the wave in the gel willnot be able to propagate through the lead 3, since its speed exceeds thecalculated maximum speed of the transmission line. In practice, therewill always be some leakage from the outer coil to the inner coil closestill causing a current in the lead tip 7.

The above example demonstrates that it is possible to reduce the energyof the propagating wave by making high inductive inner and outer coils,and high capacitive inter-coil insulation whereby the maximum allowablewave speed through the lead 3 is reduced. In practice, the capacitanceper meter for a typical commercially available lead is about 100 pF sothe 60 pF/m required is easily met. On the inductance side however, theunit inductance per meter is about 2 μH for a common 5-filar inner coil.The only way to achieve a high inductive inner coil is to make a 1-filarinner coil. This is, however, unacceptable from redundancy point ofview. Besides, the torque transfer function will not be met.

The periodic capacitance structure according to the present embodiment,i.e. the intermediate insulating tube 11 having alternating lowcapacitive sections and high capacitive sections, can serve as aband-stop filter if realized with properly chosen parameters, and therequired inductance values of the coils 9, 13 are relatively low, andthe required dielectric constants for c₀ and c₁ are reasonable. Thus,applying the periodic structure for heat reduction relaxes therequirement of high inductive coils, which would be difficult to meet.

According to an example, the lower capacitance c₀, for each lowercapacitance segment 15, was chosen to be c₀=2.5 pF, and the highercapacitance, for each higher capacitance segment 17, was chosen c₁=10pF. The length of the lead 3 was 0.52 m, and the number of segments was13, i.e. each segment 15, 17 was approximately 0.04 m long. The innercoil inductance was chosen i₀=0.5 pH, and the outer coil inductance waschosen twice thereof, i.e. i₁=1.0 pH. Twice the inductance in the outercoil than in the inner coil is a realistic relation for many kinds ofmedical implantable leads. Then a band-stop filter for a frequency rangeincluding the RF frequency of 64 MHz was obtained, i.e. the RF frequencyused in a common 1.5 Tesla MRI scanner mentioned above. The attenuationof the lead tip current was about 18 dB at 64 MHz, see FIG. 4. For acommon lead a typical value of the total capacitance is 50 pF, whichwould result in approximately 4 pF per segment, which corresponds to theexample value of the lower capacitance segments 15. In order to achievethe higher capacitance segments 17 of 16 pF each, the same material asin the lower capacitance segments 15 was doped with a dopant, moreparticularly BaTiO₄. Such a dopant increases the relative dielectricconstant of the material. In the present example, the capacitance of thehigher capacitance segments 17 is increased by a factor 4 relative tothe capacitance of the lower capacitance segments 15, which means thatthe relative dielectric constant will have to be increased by a factor16. Such an increase is easy to obtain by means of an appropriatedopant.

Referring again to FIG. 4, it shows a diagram of induced current versusRF frequency. The diagram was the result of a simulation on a circuitmodel of the above-exemplified LC structure, where the RF frequency wasscanned from 55 MHz to 75 MHz. The simulation shows a maximumattenuation, 18 dB, of the induced current at approximately 64 MHz.

In order to exemplify variation of filter parameters of the band-stopfilter, further simulations were made for inductance values of the outercoil ranging from 0.7 pH to 2 pH, and the inner coil inductance varyingaccordingly, keeping the relation between them; and for capacitancevalues of the higher capacitance segments ranging from 4 to 16 pF, andof the lower capacitance segments varying accordingly keeping therelation of 4 between them, as illustrated in FIG. 5. For example, itcan be seen that varying the inductances as well as the capacitancesaffect the outcome in terms of lead tip current at resonance frequency.

As mentioned above the section length should be about 1/10 of the RFwavelength at the lead, or shorter. It should be mentioned that also byvarying the length of the segments the filtration result is affected,that is both the tip current and the frequency of minimum current varywith a varying segment length.

According to another embodiment of the medical implantable lead 21,shown in FIG. 6, the intermediate insulating tube 25, coaxially arrangedbetween an inner coil 23 and an outer coil 27, is divided into segments29, 31, 33 having three different capacitances. Each set of segments 35consequently consists of three segments 29, 31, 33. This structure couldbe arranged to achieve an even better effect than the two capacitanceembodiment, but similarly it is more complex as regards the manufacturethereof.

According to another embodiment of the medical implantable lead 41,shown in FIG. 7, the intermediate insulating tube 45, arranged betweenan inner coil 43 and an outer coil 47, has a continuously varyingcapacitance along its length. Preferably, at least one minimum or onemaximum capacitance is positioned between the ends of the tube 45. Thecapacitance variation has been achieved by doping a basic material witha dopant such that a continuously varying concentration thereof has beenobtained along the length of the tube 45.

Above, embodiments of the medical implantable lead according to thepresent invention as defined in the appended claims have been described.These should be seen as merely non-limiting examples. As understood by askilled person, many modifications and alternative embodiments arepossible within the scope of the invention.

Thus, as explained by means of the embodiments above, by providing theintermediate insulating tube, positioned coaxially between the inner andouter coil of the lead, with a periodically alternating capacitancealong the length thereof it is possible to achieve a band-stop filterthat attenuates the RF energy transfer in the lead, and consequentlyreduces the heating of the lead caused by the RF energy.

1. A medical implantable lead, comprising an elongate coaxialarrangement of an outer coil, an inner coil and an intermediateinsulating tube arranged between the outer coil and the inner coil,wherein the intermediate insulating tube has a periodically alternatingcapacitance along the length thereof.
 2. A medical implantable leadaccording to claim 1, wherein the intermediate insulating tube comprisesa set of at least two segments, which set of at least two segments isrepeated along the length of the intermediate insulating tube, and whichsegments have different capacitances, thereby providing the periodicallyalternating capacitance.
 3. A medical implantable lead according toclaim 2, wherein the number of segments of each set is three.
 4. Amedical implantable lead according to claim 2, wherein the length ofeach segment is shorter than or equal to 1/10 of the total length of theintermediate insulating tube.
 5. A medical implantable lead according toclaim 2, wherein the segments are of equal length.
 6. A medicalimplantable lead according to claim 2, wherein the segments areindividual parts which are arranged in engagement with each other.
 7. Amedical implantable lead according to claim 2, wherein the intermediateinsulating tube is made of silicone, where at least one segment in eachset is doped with a dopant in order to provide a desired capacitance. 8.A medical implantable lead according to claim 7, wherein the dopant isBaTiO₄.
 9. A medical implantable lead according to claim 1, wherein thecapacitance is varying continuously and periodically along the length ofthe intermediate insulating tube.
 10. A medical implantable leadaccording to claim 9, wherein the capacitance variation is provided by avarying concentration of a dopant.
 11. A medical implantable lead,comprising: first and second electrodes; a coaxially configured innercoil and outer coil, the inner coil being coupled to the first electrodeand the outer coil being coupled to the second electrode; anintermediate insulating tube arranged between the outer coil and theinner coil, the intermediate insulating tube having a plurality ofsegments doped with one or more dopants to provide a periodicalternating capacitance along the length of the insulating tube.